RNA and DNA, which contain two GGAGG segments connected with UUUU or TTTT sequences, form entirely different quadruplex structures.

We determined solution structure of d(GGAGGTTTTGGAGG) (D14) in the presence of Na+ ions by NMR. Two molecules of D14 form a quadruplex strucure with parallel and antiparallel strand alignments. The quadruplex is formed by four helical GGAGG segments and two diagonal TTTT loops at the top and bottom of the helix. This quadruplex structure is entirely different from that of r(GGAGGUUUUGGAGG) (R14), which has been previously determined by NMR. In the case of RNA, R14 forms an intra-strand parallel quadruplex with a UUUU loop a t lateral position ofthe helix and two such molecules form a dimer by stacking. The factors determining the type of RNA and DNA quadruplexes are discussed.